Modeling and Simulation of Axisymmetric Stagnation Flames

The design of noise-free, low-pollutant combustion devices requires the understanding and the prediction of the combustion noise. The noise originates from the coupling between perturbations in the heat release rate with the oscillating gas (acoustic velocity). Prediction of combustion noise requires the transfer function (TF) that correlates the variation in the heat release rate of the ame with the perturbation (acoustic velocity). Computational costs prohibit an e??cient calculation of the TF by simulating all combustion phenomena associated with the ame dynamics. Simpli??ed theoretical models, such as the hydrodynamic models, allow a signi??cant reduction of the computation time and capture the main features of the ame front dynamics. The fundamental assumption of the hydrodynamic models is that the Bunsen ame, which is described as a thin layer ( ame front) separating the burnt from the unburnt gas, is attached at the burner rim. Within the thin layer approximation, the computation of the TF requires the ame area and, consequenctly, the time-dependent ame front position. By de??ning the ame front as the zero-level set of a distance function, the evolution of the ame front is described by a level set equation. The existent experiments and theoretic kinematic models based on the thin layer approximation (e.g., the G-equation model) provide contradictory information on the phase of the TF. Experiments suggest a low-pass ??lter behaviour of the gain of the TF and an increase of the TF's phase up to several ?? as the frequency increases. In contrast, most of the theoretical models indicate a saturation of the phase of the TF at the level of ??/2. Although the use of a convective wave model for the ow [80] leads to a better agreement between theory and experiments, the origin of the convective wave remains little understood. The discrepancies between theory and experiments most likely reside in the simpli??cations employed in the theoretical models. To understand the behaviour of the experimental TF, here we extend the theoretical models based on the G-equation approach. In the ??rst step, we extend the models by addressing ames that have an arbitrary cone angle and a burning velocity with variable direction. To carry out this ??rst extension we analyse the evolution of a Bunsen ame in a Poiseuille ow from an initially at pro??le to a stationary conical shape. The analysis leads to the correct derivation of the boundary conditions in the extended model employed for the ame evolution in a perturbed Poiseuille ow. We demonstrate that the new model proposed here improves the description of the front close to the boundary and, hence, the behaviour of the ame response to velocity perturbations. Because recent measurements [52] suggest that a better understanding of the TF's phase behaviour requires a hydrodynamic model that accounts for the moving edge of the ame, in the second step we extend the hydrodynamic model to account for non-attached ames. We model the ame front as an open curve by using two level set functions whose zero-level sets are locally normal at the edge point. The movement of the edge along the ame front is accounted for via a novel model for the edge speed. During computer simulations of the ame kinematics with the new extended model the local orthogonality of the level sets is preserved by employing a novel algorithm. The simulations demonstrate that our extended model qualitatively accounts for the ame stabilisation, the ame front and edge kinematics, and the stand-o?? distance of the ame above the burner rim.

Combustion instability due to thermo-acoustic interactions in high-speed propulsion devices such as gas turbines and rocket engines result from pressure waves with very large amplitudes propagating back and forth in the combustion chamber. Exposure to the pressure fluctuations over a long period of time can lead to a cataclysmic failure of engines. The underlying physics governing the generation of the thermo-acoustic instability is a complex interaction among heat release, turbulence, and acoustic waves. Currently, it is very difficult to accurately predict the expected level of oscillations in a combustor. Hence development of strategies and engineering solutions to mitigate thermo-acoustic instability is an active area of research in both academia and industry. In this work, we carry out numerical modeling of thermo-acoustic instability in a self-excited, laboratory scale, model rocket combustor developed at Purdue University. Two different turbulent combustion models to account for turbulence-chemistry interactions are considered in this study, the flamelet model and the transported probability density function (PDF) method. <br>In the flamelet modeling approach, detailed chemical kinetics can be easily incorporated at a relatively low cost in comparison to other turbulent combustion models and it also accounts for turbulence-chemistry interactions. The flamelet model study is divided into two parts. In first part, we examine the effect of different numerical approaches for implementing the flamelet model. In advanced modeling and simulations of turbulent combustion, the accuracy of model predictions is affected by physical model errors as well as errors that arise from the numerical implementation of models in simulation codes. Here we are mainly concerned with the effect of numerical implementation on model predictions of turbulent combustion. Particularly, we employ the flamelet/progress variable (FPV) model and examine the effect of various numerical approaches for the flamelet table integration, with presumed shapes of PDF, on the FPV modeling results. Three different presumed-PDF table integration approaches are examined in detail by employing different numerical integration strategies. The effect of the different presumed-PDF table integration approaches is examined on predictions of two real flames, a laboratory-scale turbulent free jet flame, Sandia Flame D and the self-excited resonance model rocket combustor. Significant difference is observed in the predictions both of the flames. The results in this study further support the claims made in previous studies that it is imperative to preserve the laminar flamelet structure during integration while using the flamelet model to achieve better predictions in simulations. In the second part of the flamelet modeling study, computational investigations of the coupling between the transient flame dynamics such as the ignition delay and local extinction and the thermo-acoustic instability developed in a self-excited resonance combustor to gain deep insights into the mechanisms of thermo-acoustic instability. A modeling framework that employs different flamelet models (the steady flamelet model and the flamelet/progress variable approach) is developed to enable the examination of the effect of the transient flame dynamics caused by the strong coupling of the turbulent mixing and finite-rate chemical kinetics on the occurrence of thermo-acoustic instability. The models are validated by using the available experimental data for the pressure signal. Parametric studies are performed to examine the effect of the occurrence of the transient flame dynamics, the effect of artificial amplification of the Damkohler number, and the effect of neglecting mixture fraction fluctuations on the predictions of the thermo-acoustic instability. The parametric studies reveal that the occurrence of transient flame dynamics has a strong influence on the onset of the thermo-acoustic instability. Further analysis is then conducted to localize the effect of a particular flame dynamic event, the ignition delay, on the thermo-acoustic instability. The reverse effect of the occurrence of the thermo-acoustic instability on the transient flame dynamics in the combustor is also investigated by examining the temporal evolution of the local flame events in conjunction with the pressure wave propagation. The above observed two-way coupling between the transient flame dynamics (the ignition delay) and the thermo-acoustic instability provides a plausible mechanism of the self-excited and sustained thermo-acoustic instability observed in the combustor.<br>The second turbulent combustion model considered in this study is the transported PDF method. The transported PDF method is one of the most attractive models because it treats the highly-nonlinear chemical reaction source term without a closure requirement and it is a generalized model for a wide range of turbulent combustion problems.Traditionally, the transported PDF method has been used to model low-Mach number, incompressible flows where the pressure is assumed to be thermodynamically constant. Since there is significant pressure fluctuations in the model rocket combustor, the flow is highly compressible and it is necessary to account for this compressibility in the transported PDF method. In the past there has been very little work to model compressible reactive flows using the transported PDF and no effort has been made to model thermo-acoustic instability using the transported PDF method. There is a pressing need to further examine and develop the transported PDF method for compressible reactive flows to broaden our understanding of physical phenomenon like thermo-acoustic instability, interaction between combustion and strong shock and expansion waves, coupling between acoustic and heat release which are observed in high-speed turbulent combustion problems. To address this, a modeling framework for compressible turbulent reactive flows by the using the transported PDF method is developed. This framework is validated in a series of test cases ranging from pure mixing to a supersonic turbulent jet flame. The framework is then used to study the thermo-acoustic interactions in the self-excited model rocket combustor.

Due to the growing concern on the issues of global warming, climate change, and energy shortage, more rigorous requirements are imposed on the energy conversion process of combustion. Due to the relative safety and convenience in transportation and storage, liquid fuels are commonly used in many practical combustion systems such as diesel engines, gas turbines and industrial furnaces. In these combustion devices, turbulent spray flows are involved. The character of spray dispersion, evaporation, mixing and combustion of fuel droplets strongly determines the performance of these systems with respect to the combustion efficiency, stability, and emissions. Therefore, an improved understanding of turbulent spray combustion and the development of predictive models are needed for a better design of more efficient combustion systems. The present work focuses on the development of a computational methodology based on the transported joint probability density function (PDF) method for the modeling and simulation of two-phase turbulent spray flows without and with chemical reactions. In the non-reacting situation, the dependent variables of the joint PDF include the gas velocity and the mixture fraction. For the simulation of reacting spray flows, a three-variate joint PDF transport equation is derived and modeled in order to account for the pre-vaporization effects and partially premixed regime in turbulent spray flames. The dependent variables include the gas phase mixture fraction, the reaction progress variable and gas enthalpy. Detailed combustion chemistry is considered through an extended spray flamelet model by including a reaction progress variable in addition to the classical formulation. The dilute spray is simulated using a Lagrangian discrete parcel method for the description of droplet motion, heating and evaporation. The infinite conductivity model with consideration of non-equilibrium effects based on the Langmuir-Knudsen law is considered. The spray evolution and flame structures in the frame of the polydisperse reacting spray flows are investigated. Numerical results are compared with experimental data provided by Prof. Masri at the University of Sydney, Australia, and the test cases include three different turbulent non-reacting acetone spray flows in air and turbulent spray flames with the liquid fuel ethanol. For the acetone spray flows, computational results generally show good agreement with experimental data in terms of the droplet size and mean velocity distribution, as well as the liquid volume flux. The results show that the inflow liquid mass loading hardly affects the droplet diameter distribution, whereas the inlet turbulence level has a pronounced effect. The tendency of droplet accumulation near the jet centerline is found with a somewhat overprediction of liquid volume flux at downstream locations. A more sophisticated turbulence model is expected to eliminate this discrepancy. Moreover, the local joint PDF of the gas velocity and the mixture fraction is analyzed. A linear correlation of the gas velocity and the mixture fraction exists close to the nozzle exit outside the main spray jet, and no regions are found where statistical independence prevails. For the simulation of spray flames, computations with the newly developed spray flamelet/progress variable approach and with the previous standard spray flamelet formulation are carried out and compared with the experiments by Prof. Masri at the University of Sydney. A good agreement between the computations with the new formulation and the experiments for gas temperature and droplet size and velocity is achieved. The major spray and combustion properties are correctly captured using this new formulation, which is compared with an unphysically attached flame near the nozzle exit predicted by the previous model. A partially premixed combustion prevails in this piloted turbulent spray flame. Due to the prevaporization of the ethanol droplets near the nozzle exit, a lean premixed gas mixture is found at the inner side of spray jet. Moving downstream, the lean-sided diffusion flame is promoted towards the inner fuel-rich side by heating up the inner premixed core that is controlled by the droplet evaporation. Additionally, it is observed that in the far-field region, the diffusion flame becomes the dominant combustion mode. In summary, an efficient computational model based on the transported joint PDF method is developed to two-phase turbulent spray flows. The combined transported joint PDF and a newly proposed spray flamelet/progress variable approach shows an improved performance in the prediction of complex turbulent spray flames, and new insights on the local flame structure influenced by evaporating sprays are obtained.

Extensions to the Front

In order to investigate the effects of cigarette circumference on combustibility and smoke aerosol properties, the solid phase temperature of cigarette combustion cone and gas phase temperature in filter tip of the same brand cigarettes with three circumferences (O24.37 mm, normal; O20.06 mm, medium; O17.02 mm, slim) were tested by infrared thermal temperature and thermocouple temperature measurement technologies, respectively. A smoking cycle simulator and a fast particle spectrometer were employed to determine the particle size of mainstream smoke aerosol under ISO smoking regime. The results showed that: 1) The solid phase temperatures of the medium and slim cigarettes were higher than those of the normal cigarette by ~100℃ on average under puffing. 2) The gas phase temperatures at the filter front of the medium and slim cigarettes were higher than that of normal cigarette by ~5℃. However, the gas phase temperatures in the filters of the three sizes of cigarettes tended to be consistent. 3) With the decrease of cigarette circumference, the average particle number concentration of aerosols for all puffs increased significantly, while the count median diameter (CMD) decreased, which indicated that the number of fine particles increased. 4) With the decrease of circumference, the average particle surface area of aerosols for all puffs increased significantly, and the area median diameter (AMD) reduced, which suggested that the total area of fine particles increased significantly.

One hundred and ten (110) streams have been identified in an oil refining process as contributors to the gas fuel net used in furnaces and boilers to supply power and steam to process. According to the origin of the gases and the previous treatment, these gas streams, have a different composition than the natural gas, which creates problems that affect the operational parameters and the integrity of equipment. It is due to this, which seeks to analyse how the fuel gas flows from a refining process, taking into account the energy efficiency, environmental and safety aspects in combustion process. Statistical analysis was developed in combustible gases to study their composition considering the origin of each of the streams represented by chromatographic analysis, which, provided the data for calculating the methane’s number (MN) via method radius ratio (H/C), and the Wobbe index as criteria for analysing the potential effect of gas fuel composition in efficiency, integrity and environmental emissions equipment. The methane’s number is a parameter which describes the behaviour of the explosive self-initiated and not controlled combustion in gas zone of the front flame (referred as knock Combustion). This knock resistance index, has two reference gases, methane and hydrogen; using a reference fuel mixture in which the methane works with a MN 100 and hydrogen with a MN 0. High numbers of MN means high efficiency and hence lower CO2. If MN is too low, knock can cause damage or loss in efficiency and performance. The results show that 45% of the streams of petrochemical area presented MN above 80 and 15% of streams presented low MN. Streams of cracking area showed lower MN. These results demonstrate the heterogeneity of currents, so that the methane number is an important reference when defining the logistic for combustible net in combustion process. Elaborating further on the effects that this variability can generate in the thermal efficiency of each of the currents, so as combustion efficiency and the stability of the operation was evaluated Wobbe Index, and the results showed that when applying these two methods and taking into account the current regulations, a comprehensive analysis can be performed of energy issues, environmental and security in combustion process of fuel variable composition. The principal result was a proposal for a fuel net.

The feedstocks of natural gas exceed the resources of crude oil by far. Therefore, there is a strong interest in the utilisation of natural gas with its main component methane for the production of more useful chemicals. This leads to a strong demand for compact and low-capital-cost reactors for the conversion of natural gas to methanol and liquid hydrocarbons and the production of H2. The combustion of natural gas is also considered for power generation in catalytic devices, because the flameless catalytic process results in lower emissions of NOx. Noble metals such as Pt, Pd, and Rh are very active catalysts for the conversion of CH4. However, the processes occurring during the partial and complete oxidation of CH4 on these catalysts are not completely understood. A detailed understanding of the reactor behaviour at all possible conditions is crucial for the technical realisation of these catalytic processes, because explosive mixtures are handled. In particular it is necessary to control the operation of the reactors at transient conditions such as light-off and shut-down. The objective of this work is to study total and partial oxidation of methane over Pt, Pd, and Rh catalysts for a wide range of operating conditions. Therefore, an experimental setup, which is easily applicable for modelling and numerical simulation, has to be developed. The experiment and analysis has to be arranged in a way that allows studies of transient processes such as light-off. Based on the experimentally derived conversion and selectivity surface-reaction schemes, available in literature, have to be evaluated. Available computational tools have to be used for numerically predicted reactor performance and compare these results with the experimental data. Crucial conditions, at which the reaction models need improvement, have to be found, and if necessary and possible improved. A flow reactor with associated analysis is designed to meet these requirements. In order to facilitate the simulation of the system catalysts exhibiting a simple geometry such as monolithic structures are used. The reactor consists of a 40 cm long quartz tube with varying inner diameters. Honeycomb catalysts coated with Rh, Pd, or Pt are placed inside. The gas temperature at the exit of the catalytic monolith and outside the quartz tube is. The reaction is ignited by heating up the reactor by a furnace, in which the flow system is integrated. After ignition, in the case of autothermal operation, the furnace can be switched off. The gases are premixed at room temperature and flow into the quartz tube at atmospheric pressure. The product composition is quantitatively analysed by a quadrupole mass spectrometer (QMS) in order to study transient phenomena. For the evaluation of reaction mechanisms, detailed numerical simulations of the physical and chemical processes using the computer program package DETCHEM and the commercially available CFD code FLUENT are applied. The numerical codes take into account detailed mechanisms of surface and gas-phase reactions as well as mass and heat transport processes in the channels and heat transport in the solid monolithic structure. The channels are simulated under steady-state conditions in a 3D elliptic approach using FLUENT and in two dimensions by a parabolic approach using DETCHEMCHANNEL. Transient phenomena are simulated by DETCHEMMONOLITH with 2D parabolic flow field simulations of a representative number of single channels including transient heat balances of the monolithic structure. First, the ignition of catalytic combustion of CH4 on a Pt loaded monolith is investigated. The conversions of CH4 and O2 as well as the product selectivities are experimentally determined and the results numerically simulated. The influence of H2 addition on the combustion is investigated. Reaction-rate oscillations of the catalytic combustion of methane on a Pd based catalyst are found experimentally at lean conditions in Pd coated monoliths and simulated with a surface-reaction mechanism. The second set of experimental focuses on CPO of CH4 over Rh coated monoliths. The experimentally derived conversion and selectivities are used to improve a detailed reaction mechanism. Next, the influence of the support material on the ignition process is investigated. In the experiment, either -alumina or cordierite as support material is applied and coated with Rh. The models are able to describe the light-off behaviour and are applied for catalyst design: The ignition behaviour of a set of virtual catalytic monoliths exhibiting varying physical properties is modelled. In addition, catalysts coated with Rh-nanopowder produced by laser ablation is experimentally examined and compared with wet coated catalysts. Finally, the CPO of methane is carried in a Pt, presenting an even more well-defined configuration for modelling. At CH4/O2 ratios above 1.9 complex patterns of oscillation in conversion and selectivity occur.

Methanation of biosyngas in a fluidized bed reactor

Chemical kinetics modeling and LES combustion model effects on a perfectly premixed burner

Tabulated chemical kinetics for efficient and detailed simulations of diesel engine combustion

The world is steadily moving toward greener alternatives of established chemical production processes. Process modeling and simulation play a vital role in conceiving, designing and optimizing these greener processes. In many cases, these simulation tools are limited to a few project design stages, thus limiting their true potential. This thesis presents the benefits of process modeling and simulation at different levels of process development. The work focuses on ammonia production, which sustains global agricultural and mining activities. Ammonia production has traditionally been based on large-scale plants. The thrust toward large-scale production in order to gain economic advantages has overshadowed the benefits that could be derived from small-scale production plants. Additionally, the ammonia industry is a major consumer of global fossil fuels, and in the process of providing ammonia, also burdens the planet with greenhouse gases. In order to ensure sustainable ammonia production, this thesis highlights the techno-economic advantages that result from small-scale ammonia plants based on biomass feedstock. The use of biomass as a potential feedstock can help neutralize the carbon footprint of ammonia production. To establish the feasibility of such a process, this thesis presents the application of process simulations at different modeling levels.<br> <br> A Computational Fluid Dynamics (CFD) model was used to understand the flow patterns inside a dual fluidized bed gasifier. This elevated the understanding of the hydrodynamics of the gasifier freeboard, which is neglected by the conventional two-phase methodology. The CFD simulation was utilized to perform a Residence Time Distribution (RTD) analysis of the reactor. Four different tracer approaches namely the frozen velocity approach, the snapshot approach, the data sampling approach and the transient approach, were compared. The RTD analysis formed the basis of a steady-state compartment model that was developed in ASPEN Plus simulation software. The ASPEN Plus gasifier model decoupled the pyrolysis, gasification, and combustion sections of the gasifier to affect a better comprehension of the process and the results. The model predicated satisfactory results upon validation. Additionally, the model could also be used to predict the output for different biomass feedstocks.<br> <br> ASPEN Plus is used to simulate the biomass gasification process as well as downstream gas conditioning that leads to ammonia production. Apart from major constituent gases, the model accounted for tar production in the gasifier process as well as for its subsequent removal. Three different configurations of the biomass-to-ammonia process, namely, Autothermal Reforming (ATR), Steam Methane Reforming (SMR), and CO₂ Reforming (CR) have been modeled and compared. The configurations differ on the basis of the reforming of the methane and tars, which are present in the syngas. The output of the ASPEN Plus simulation is fed into an MS Excel-based framework, which performs Life Cycle Costing (LCC) and Life Cycle Assessment (LCA) for the simulated flowsheet. Finally, these outputs are utilized by an MS Excel-/VB-based Multi-Objective Optimization (MOO) framework in order to optimize both LCC and LCA for changing process parameters.<br> <br> Three biomass feedstocks, namely, wood, straw pellets, and bagasse, which are available in Australia, India, and Brazil, respectively, have been compared in order to understand the effect of biomass composition, the supply chain, and the national economic and environmental scenario on the viability of the biomass-to-ammonia process. The MOO results that were derived for changing flowsheet configurations as well as for different biomass feedstocks that were grown at different locations worldwide are expected to highlight a more rigorous analysis of the biomass-to-ammonia process. Such an analysis will provide decision makers with consistent and comparable data to objectively judge the viability of the proposed process. This analysis distinguishes this thesis from past studies that focused on ammonia production from biomass, and presented their LCC and LCA results for a single process configuration and for a single feedstock. The results predicted that different biomass feedstocks, process configurations, and geographical regions have their own advantages. The results provide a benchmark for comparison of other biomass-to-ammonia processes on a techno-enviro-economic scale. The methodology can be employed as an example in the development of different sustainable chemical production processes.

The desire to meet rising energy demands while simultaneously adhering to stringent climate policies has propelled advanced combustion technologies and alternative fuels. Natural gas (NG) is an attractive, domestically-available alternative that has been shown to achieve high efficiencies when used on dual fuel engines. However, the composition of NG can vary significantly depending on the source and these variations may detrimentally impact engine operation. To study these effects, a dual fuel engine simulation model was developed and calibrated against experimental data obtained from a heavy-duty 6-cylinder dual fuel compression ignition engine. Different NG compositions were studied on this simulation model to analyze the impact that composition plays on the combustion process in a dual fuel engine architecture. The results show that composition can play a significant role in the combustion efficiency, peak pressure, and combustion phasing. These impacts were studied in a constant equivalence ratio case in which differences in stoichiometric air-fuel ratios were taken into account. The results reveal that if variations in stoichiometric air-fuel ratio are accounted for, blends with high methane contents and low nitrogen behave similarly.

In the present study, investigations were carried out to obtain data on combustion characteristics of methane gas and hydrogen supplemented methane gas in a constant volume combustion chamber. The main results obtained from the study can be summarized as follows. The maximum combustion pressure increases as the initial pressure and hydrogen supplement rate increase, the total burning time is shorten by lowering the initial pressure and by increasing the hydrogen supplement rate. The maximum flame temperature and NO concentration increase by the initial pressure and hydrogen supplement rate increase. The flame propagation processes in near stoichiometric mixture are propagated with a spherical shape, but in excess rich or lean mixtures are propagated with a elliptical shape. And, they are changed an unstable elliptical shape flame with very regular cells by increasing the hydrogen supplement rate.

Before correction After correction p.158 4.2 () (EDT) 3 10 229.3 , 30 . (ISO) 3 47 224.2 . 4 . 15 Level 2 600 . (EDT) 7 50 600

In the present thesis, it is attempted to build a bridge between the most simplified systems that have already been studied in literature (laminar flame burners, in which simple fuels such as methane, ethane or ethylene are burnt at atmospheric pressure) and the much more complex and demanding environment that is found in practical combustion engines. This is accomplished by designing and constructing a high pressure vessel and laminar burner (HPVB) integrated with an evaporation system. The capability of the HPVB setup of burning vaporized liquid fuels in laminar diffusion and partially premixed flames brings the important opportunity to isolate more easily the impact of fuel chemistry on the combustion behavior of relevant fuels and biofuels. This experimental setup is particularly designed to offer optical accessibility for laser diagnostic techniques, also allowing their assessment and development at elevated pressures. Background information on the type of burners mostly used for combustion studies in laminar flames is given in chapter 2 with focus on measurements in a high pressure environment. A detailed description of the design capabilities of the HPVB setup and the flame stability issues encountered are also presented and discussed. The theory behind the laser diagnostic techniques applied in this work are reviewed in chapter 3. The chapters 4 to 6 present the measurements carried out to characterize a large range of flames from gaseous and vaporized liquid fuels using laser diagnostics. In chapter 7, the heat flux method for laminar burning velocity measurements is presented and a feasibility study is performed to extend the applicability of the method to higher pressures. This is accomplished by integrating the heat flux burner in the high pressure vessel of the HPVB setup. Chapter 8 summarizes the conclusions and provide recommendations.